I'm probably being unpopular writing this, but I never quite liked the 555. Not diminishing its value and the ingenuity that went into its design; I rather found myself much more attracted by CMOS gates, also in analog circuits. One day I was playing with PLLs and to better understand how phase detectors work, I built a proof of concept motion detector off a single quad xor chip (4030 probably), 1st gate working as ultrasound oscillator connected to a tx capsule, 2nd one biased as linear amplifier with input connected to a rx capsule, 3rd gate working as phase detector taking both 1 and 2 gates outputs, 4th gate driving a LED, which would flash every time I moved the hand in front of the capsules as I was delaying one of the two signals, which triggered the gate.
Very fun and instructional. The interesting part is that a CMOS digital gate can become a decent linear amplifier if properly biased, not unlike more common opamps, so where high linearity or fidelity in audio signals aren't a must, it can be quite an interesting part.
As an example, here's a equalizer built around a 4049 quad inverter gate chip.
http://www.runoffgroove.com/mreq.html
As for digital gates, I couldn't recommend more the TTL and CMOS cookbooks by Don Lancaster: they're a goldmine of ideas. 2nd one is available for free at author's site.